The present invention relates to aircraft electric power distribution systems (EPDS) and methods for improving the lightning immunity thereof, and more specifically, to EPDS and methods for preventing undesirable trips of a solid state power controller (SSPC).
An aircraft EPDS is often required not only to survive without any physical damage when lightning strikes the aircraft, but also to remain operational during and after the strike. A typical requirement for an EPDS would likely be stated as “the system shall not change the state of any SSPC channel when subjected to the lightning and EMC environment”. This requirement poses a significant challenge to the designs of SSPC based EPDS, as an SSPC often contains electronic circuitry which could be damaged or upset by the excessive transient voltages induced by the lightning and SSPC could result in undesirable (or nuisance) trips due to lightning strikes. The increasing use of composite materials, instead of aluminum for weight and strength benefits in the aircraft fuselage has only made the situation worse.
The “nuisance trip” mechanism can be explained by FIG. 1, where there is shown a graph 100 of current versus time. The graph 100 shows a typical SSPC trip curve 102 such that, when the current at the SSPC is greater than that of the trip curve 102, the SSPC will trip to an off state. During a lightning strike, if the SSPC were in the turn-on state, the induced surge current could reach at a point 104, which is far exceeding the trip cure 102 limit and therefore will cause SSPC to trip. In order to avoid this nuisance trip, the SSPC switch would have to be operated in the current limiting mode, so that the induced current passing through the SSPC can be controlled (limited) at a point 106, which is below the SSPC trip curve.
Transient voltage suppression (TVS) devices are usually used to clamp the lightning transient voltage wherever necessary, which ensures no damage to the electronic circuitry. However, the use of TVS does not prevent (if not potentially increase the chance of) the undesirable (or nuisance) trips in those SSPC channels in the turn-on state when lightning strikes, due to high transient current as result of the lightning (as shown at point 104 in FIG. 1), which would trigger the instantaneous trip mechanism designed for SSPC short circuit protection, causing power interruption to the corresponding aircraft loads connected. Once these SSPC channels are tripped off, they are usually not allowed to be turned on again during the remaining flight to avoid potential fire, as the conventional SSPC cannot tell whether the trip is due to an actual over current fault or a lightning strike.
One prior art, US 2008/0106152 A1, tries to address the “nuisance trip” issue by monitoring the voltage across the switch of the SSPC together with the current passing though an SSPC channel in the turned-on state, using a micro-controller that serves as the trip engine to determine whether to put the SSPC switch in a current limiting mode (for lightning) or to trip (turn SSPC off for over current fault). However, the feasibility of the method proposed in this prior art is yet to be verified. An excessive voltage across the SSPC switch however, as is monitored in the '152 publication, is not a necessary indication of the lightning (switching off an inductive load with a nominal loading or a faulty over current will also results in voltage spikes across the switch).
Another related prior art, US 2008/0129113 A1, tries to address the “nuisance trip” issue at the system level, which at the most, would only reduce the number of “nuisance trips”.
Referring now to FIG. 2, there is shown a typical alternating current (AC) or direct current (DC) SSPC channel 110 with conventional lightning protection. Each SSPC channel 110 mainly comprises a high power solid state switching device (SSSD) 112 for the main power distribution function, a SSSD driver (or gate driver) 114 that turns the SSSD 112 “ON” or “OFF”, and a DSP based SSPC processing engine 116. Two TVS devices 118a, 118b, each connected with an EMI capacitor 120a, 120b in parallel, are connected at both ends of the SSSD 112 to suppress the potential voltage surge due to lightning coming from either the power source side 122 or the load side 124.
The SSPC processing engine 116 is mainly responsible for current sensing signal processing, SSSD on/off control and feeder wire protection. It generates proper gate drives for the SSSD 112 to provide required power commutation according to received command during normal operation. The SSPC processing engine 116 also turns off the SSSD 112 according to either the thermal energy level inside the feeder estimated using the current sense signal from a current sensor 126 (through the over current trip block 128), or the absolute current signal amplitude when it exceeds a predetermined (instantaneous) trip level. The instantaneous trip level is used to avoid passing a faulty current higher than this level (e.g. in a short circuit fault situation) for unnecessary long period of time, which could over stress the SSSD 112. The instantaneous trip is realized by converting the instantaneous current sensing signal into a conditioned (rectified) voltage signal and comparing it with a preset voltage reference VRef. If at any time the rectified voltage signal exceeds the reference value, an active interrupt signal will be generated by the comparator block 130 causing an interrupt in the DSP 132. The corresponding interrupt routine will then log an active instantaneous trip status (at instantaneous trip block 134) inside the DSP 132 and then turn off the SSSD 112. As can be seen, it is this instantaneous trip mechanism that could mistakenly take the current surge passing through the current sensor due to lightning as a short circuit fault, and consequently removing the power to the connected load. Therefore it is crucial to find a simple and effective way that can distinguish between these two situations.
As has been mentioned above, the TVS devices 118a, 118b in FIG. 2 are used to provide lightning protection to the SSPC channel. When excessive lightning induced surge voltage “hits” an SSPC channel from either the power input side or the load output, one of the TVS will be forced into the break-down state, diverting significant amount of surge current through the TVS.
As can be seen, there is a need to provide a solution for the SSPC to effectively distinguish between the current surge as a result of lightning and the actual over current fault, to avoid the “nuisance trips” in the presence of lightning.